Spectrophotometric analysis is a method of chemical analysis based on the absorption or attenuation by different substances of light of a specified wave length. The instruments used to perform such an analysis are referred to as spectrophotometers. The sequential dual wave length spectrophotometer uses a common light path to cause two light beams to be transmitted through the sample sequentially at two different wave lengths, one of the light beams being the reference beam (I.sub.R) which establishes reference thresholds for the other beam (I.sub.S).
There is a need in spectrophotometers to determine the sensitivity of the instrument and to correct for errors introduced into the spectrophotometer readings due to imperfections in the components which make up the system. One error which arises is due to non-linearities in the spectrophotometer detector. Photomultiplier tubes, which usually constitute the spectrophotometer detector, for example, become non-linear at large anode currents. Another error is caused by off-sets in the detector. For example, the dark current in a photomultiplier tube adds a fixed amplitude to its output signal. In addition, the electronic circuitry which is used to sense and process the output signal from the detector often has inherent non-linearities and zero off-sets which create photometric reading errors.
The principal object of the present invention, as stated above, is to provide a suitable system for calibrating spectrophotometers so that the aforesaid scaling and linearization errors may be compensated.